Acute intermittent porphyria (AIP) is an inherited metabolic disease characterized by a deficiency of porphobilinogen deaminase (PBGD), the third enzyme of the heme synthesis pathway. The enzyme activity is ˜50% of normal in those who inherit the genetic trait. The disease is inherited in an autosomal dominant manner and is the most common of acute porphyrias. Although it occurs in all races it is most prevalent in North Europe, mainly in Sweden, Britain and Ireland. In USA and other countries the estimated prevalence is 5/100,000 and in Northern Sweden it is as high as 60-100/100,000. More than 225 mutations in the PBGD gene have been described to date. The dominant clinical feature is an acute intermittent attack due to dysfunction of the nervous system, including abdominal pain and neurovisceral and circulatory disturbances. Abdominal pain has been reported in 85-95% of cases and is the most common feature, followed by or associated with the neurological changes. Progression to respiratory and bulbar paralysis and death may occur if AIP is not recognized and harmful drugs are not withdrawn, such as drugs metabolized by the hepatic cytochrome P450 enzymes which may precipitate an attack. Sudden death may also occur as result of cardiac arrhythmia. Primary liver cancer and impaired renal function sometimes occur as well.
An inherited deficiency of PBGD is not enough for the symptoms to appear. A high proportion of subjects that inherit PBGD mutation never develop porphyric symptoms, i.e. there is very low clinical penetrance. Clinical symptoms in AIP carriers are associated with increased production and excretion of the porphyrin precursors delta-aminolevulinic acid (ALA) and porphobilinogen (PBG) as result of increased demand of heme synthesis due to a drug or other precipitating factors that provoke the acute attack. In these conditions PBGD deficiency limits heme synthesis and as a result heme-mediated repression of ALA synthetase (ALAS1) is impaired. There is evidence indicating that the liver is the main source of the excess of porphyrin precursors. These compounds remain elevated between attacks in those subjects prone to repeated porphyric crises and increase further during the crisis. They may decrease to normal if the disease remains inactive for a long period of time.
Acute attacks usually occur after puberty and can be induced in latent individuals by endocrine factors and steroid hormones and a variety of environmental factors including drug, nutritional factors, restricted carbohydrate and caloric intake, smoking, steroid hormones and oral contraceptives, lead poisoning, intercurrent infections, surgery and psychological stress. Drugs are among the most important factors that precipitate acute attacks and a list of safe drugs is available in www.drugs-porphyria.com. Smoking, ethanol and drugs metabolized by CYP450, greatly increase hepatic heme demand and result in the induction of ALAS1, which increases the production of porphryin precursors and precipitates an acute attack. Also, ALAS1 is positively regulated by the peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), which is induced in the liver during fasting. Among the precipitating factors steroid hormones seem to play an important role. This concept is supported by the fact that the disease rarely manifests before puberty and that oral contraceptives can exacerbate attacks in some females with PBGD deficiency. Also women (80%) are affected more often than men (20%).
Acute attacks are treated with infusions of glucose and hemin (Normosang, Orphan Europe). Glucose appears to antagonize the ALAS1 induction mediated by PGC-1α. Hemin restores the regulatory heme pool and suppresses hepatic ALAS1 induction. Some women develop premenstrual attacks which can be prevented by gonadotropin-releasing hormone (GnRH) analogs. Some patients exhibit recurrent acute attacks and significant, disabling neurological dysfunction. Advanced neurologic damage and subacute and chronic symptoms are generally unresponsive to heme therapy. This is a life-threatening condition that can be cured only by allogeneic liver transplantation that, in three patients to date, prevents the accumulation of neurotoxic ALA and PBG. Nevertheless liver transplantation has limited availability of compatible donors, and a significant morbidity and mortality.
Thus, gene-replacement therapy is a potential alternative to liver transplantation in these patients where the liver function is entirely normal except for the PBGD deficiency. Gene therapy is a procedure consisting of the introduction of a specific gene into cells to control disease through the use of vectors. The feasibility of gene delivery therapies aiming to correct the hepatic enzyme defect are being explored in experimental models of AIP (AIP mice). Adenoviral vector-mediated gene transfer of PBGD to porphyric mice revealed short-term therapeutically efficacy as a result of the transient hepatic expression of PBGD (Johansson, 2004, Mol. Ther. 10(2):337-43). These results established a proof-of-principle, demonstrating that viral vector-mediated PBGD gene delivery may transiently ameliorate the severe manifestations of phenobarbital-induced porphyric attacks in AIP mice.
EP 1 049 487 discloses the construction of rAAV vectors containing a human PBGD cDNA only at a conceptual level.
There is however still a need in the art for improved vectors and protocols for AAV-mediated delivery of hPBGD to subjects in need thereof.